What Consensus Mechanism Is an Alternative to Proof-of-Work (PoW)?

Proof-of-Stake (PoS) is a major alternative to Proof-of-Work. In PoS, block validation is determined by the amount of cryptocurrency a validator holds and is willing to "stake" as collateral, rather than by computational power.

This mechanism changes the attack vector; a 51% attack in PoS would require an attacker to acquire 51% of the total staked coin supply, making it an economic attack rather than a computational one. PoS is generally considered more energy-efficient.